The process of taking a database system “off-line” is counter-productive to the goal of maintaining the database availability for 24 hours a day, 7 days a week and 365 days a year. Currently, with the present types of database systems, a database system must be taken off-line to create a physically consistent snapshot for the purpose of offloading database processing in a mirrored disk environment. The present invention provides for the capability of creating a “physically consistent snapshots” while the database system still remains on-line.
Users of data processing systems who are required to maintain large volumes of data must make provisions to ensure that backup data is available in case of some disaster or power shutdown. For example, banks, insurance companies, stock market traders, financial institutions, who require large databases would be put into a difficult position should a failure occur which would render the database unavailable.
Thus, data processing system users will be seen to arrange to maintain copies of their valuable data on the same site or on some other storage media, or as in the present situation, make use of a secondary “mirrored” storage device which is located in communication with the main storage device. Thus, should some problem or disaster occur, then both the primary storage of data, as well as the secondary or backed-up data storage (which would ordinarily be unavailable to the host), could later be made available through the remote storage of the backup data.
One problem that occurs in prior art data storage systems, is where they use a method of data mirroring, wherein one host CPU or processor writes data to both the primary storage device, as well as a secondary data storage device. However, this leads to problems because this overly burdens the host CPU with the task of writing data not only to the primary data storage system, but to the secondary data storage system, and therefore can reduce system performance.
In the present invention, there is indicated a method of creating a physically consistent database from an on-line database system for the purpose of capturing a physically consistent mirrored snapshot of the database. A logically consistent database is maintained for an on-line database system by reading data from physical disk storage and temporarily storing the data in active memory storage. The data stored however, in memory, can undergo many changes that are activated by on-line database applications. These changes result in a database that is “not physically consistent” while the database system remains on-line. Thus, it can be understood that—the only method of securing a physically consistent database is to take the database system off-line. Physical consistency of a database is most desirable in a mirrored-disk environment, whereby mirrored copies can be split from their original source. The mirrored copies represent point-in-time snapshots of a physically consistent database. These snapshots can be used to off-load processing from the “original” database system thereby improving performance of the original system. Thus, by creating a physically consistent database from an online database system, there would be no need to take the primary system off-line, whereby there would be an increased database availability.
The present invention relates to the method of creating an external physically consistent database from an on-line database system. An on-line database system maintains a logically consistent database by (i) reading data from disk storage into a system memory storage, then (ii) making changes to data by updating system memory storage, and then (iii) writing the changed data to disk storage at periodic intervals. When there are no active users of a database system, all the modified or changed data is written from the system memory storage to the disk storage, and then the system can be taken off-line with the database in a physically consistent state. However, with the advent of physically mirrored disk storage, the database system process of off-loading of a physically consistent copy is enabled and the performance can be improved, whereby the primary system remains available for normal operations and the secondary system is available for backup operations.
As previously noted, the process of taking a normal database system off-line is counter-productive to the goal of maintaining 24 hour, 7 day a week, 365 days a year of database availability. Thus, if a physically consistent database could be created from an on-line database system, the database availability will still be maintained while the system performance is improved when a physically mirrored snapshot is used to off-load the processing.
An example of such a remote mirroring system is illustrated in U.S. Pat. No. 6,044,444 to Yuval Ofek of the EMC Corporation. This involves a data processing system which automatically and asynchronously, with respect to a first host system, generates and maintains a backup or “mirrored” copy of a primary storage device at a different location physically remote from the primary storage device. This is done without any intervention from the primary host which might seriously degrade the performance of the data transfer link between the primary host computer and its primary storage device.
U.S. Pat. No. 6,044,444, provides a method of mirroring physical storage in order to create a duplicate copy, described as a “physically mirrored snapshot”. However, this U.S. patent to Ofek of EMC Corporation does not teach or show any method that provides physical consistency in the duplicate copy.